Ferroelectric RAM
As Ferroelectric Random Access Memory (FRAM or FeRAM ) refers to a non-volatile electronic memory type on the basis of crystals with ferroelectric properties, ie the ferromagnetism analog electrical properties.
The structure corresponds to that of a DRAM cell, a capacitor having a ferroelectric dielectric is used instead of a conventional capacitor. Ferroelectric materials possess a permanent electrical polarization even without external electric field is similar to ferromagnetic materials. By an external field, this polarization can be " switched " in a different direction, whereupon the storage mechanism of the FRAM based.
A ferroelectric material commonly used is the crystallizing in perovskite - type barium titanate, BaTiO3. The positively charged titanium ions align themselves to one side of the crystal, while the negatively charged oxygen ions align to the opposite side, resulting in a dipole moment and thus the permanent polarization of the whole crystal results.
Production
On a semiconductor wafer, a ferroelectric thin film is applied which replaces the gate insulator of the usual memory cell FET. The layer consists of, for example, lead zirconate titanate (PZT) or chemically Pb ( ZrxTi1 -x) O3 or SBT ( SrBi2Ta2O9 ). The storage and erase operation is realized by a change in voltage and thus a change in the field and polarization of the ferroelectric layer.
Circuit variations
There are several circuit variations for ferroelectric memories. The most common are the so-called 1T1C and 2T2C cells, each with one or two transistors and capacitors with ferroelectric dielectric. The transistor in each case for selecting the memory cell needs to be described, since ferroelectrics do not have a crisp switching voltage, but the Umschaltwahrscheinlichkeit increases with the magnitude of the voltage and the duration of the voltage pulse. Cells are described by, after selection of the desired memory cell via word lines and bit lines, the polarization of the ferroelectric is set by a voltage pulse.
Read the memory cells are also on the voltage pulses, wherein the memory cell is described having a defined state. Depending on whether this reverses the direction of polarization, or remains the same, results in a different displacement of current, which is controlled by a sense amplifier ( sense amplifier Sheet ) converted into a corresponding voltage signal on the bit line. Since this read operation destroys the existing polarization cell has to be written to the original memory contents thereafter.
Furthermore there are 1T cells only from a ferroelectric field effect transistor ( FeFET ) consist. At a FeFET the gate insulation is replaced by a ferroelectric dielectric (analogous to the floating gate in the flash memory ). By the electrical polarization of this dielectric, the current -voltage characteristics of the source- drain junction is influenced: Depending on the polarization direction, the transistor switches on, or continuity. The FeFET is described by applying an appropriate voltage between gate and source. The FeFET is read by measuring the current when a voltage between source and drain. The read operation is not destructive.
Properties
- Non-volatile (unlike DRAMs)
- Compatible with the common EEPROMs
- Data is retained for 10 years, even with severe temperature fluctuations
- 1010-1015 read and write cycles guaranteed (depending on type and manufacturer )
Life and causes of failure
FRAM also become unusable after a certain period of use. High temperatures accelerate this decay. There are two possible causes of error: depolarization and imprint. In depolarization falls with time, the polarization level of the cell. Thus, the current state is no longer detected correctly after a certain time. This effect increases at high temperatures. In addition, the cells must not be operated above the Curie temperature of the ceramic, as otherwise the characteristics of the ferroelectric ceramic, by the change of the spatial structure disappear.
Imprint means that the cell is trapped in a polarization state, the write pulse is no longer sufficient to cause a polarization reversal.